Stroboscopic cathode-ray oscilloscope system for comparing two frequencies



p 1947- R. T. ADAMS STROBOSCOPIC CATHODE RAY OSCILLOSCOPE SYSTEM FOR.COMPARING TWO FREQUENCIES Filed Feb 8, 1944 T? T VAR/ABLELOW F FREQUENCYr= CAL IBRATED l s v OSCILLATOH FIG. 2

I 7 135 U U s s .s 8 u u i s INVENTOR By R. r ADAMS W h 77; m ATTORNEPPatented Sept. 2,1941

' NlT STROBOSEOPEC CATHQDE-RAY OSCELO- SCOPE SYSTEM FOR (DQARING TWOFREQUENCES Robert '1'. Adams, Baltimore, are... assignor to WesternElectric Eompany, Incorporated, New York, N. K, a corporation of NewYork Application February 8, 1944i, Serial No. 521,568

4 Claims. (on. re-e45) In the prior art frequencies have been com paredby causing them to produce stationary,- reentrant patterns known asLissajous figures.

Where thefrequency ratio is a harmonic ratio which may be represented bysmall whole numbers, this method is easy to use. However, where thefrequencies are nearly equal and in a ratio which must be represented bylarge whole numbers, the patterns produced often get so compllcated asto become worthless. Nevertheless the simplicity of the directcomparison method is very desirable.

It is the object of this invention to provide a method of and a meansfor directly comparing Fig. 2 is a series of Lissajous figuresillustrating the principles of the invention;

Fig. 3 is a Lissajous-figure which may'result when two frequencies arenearly equal and their ratio is represented by relatively largeintegers; and,

Fig. 4 shows a simpler pattern derived from the pattern of Fig. 3 inaccordance with the teaching of this invention.

In Fig. 1 a standard source of frequency l is applied to one pair of thedeflector electrodes t of a cathode ray oscilloscope 3 while an unknownfrequency source 2 is applied to the other pair of electrostaticdeflector plates of the same oscilloscope. Oscilloscope 3 includes inaddition to the electrostatic deflector, plates 6 an electron gun whichmay comprise an indirectly heated cathode I, an intensity controlelectrode 8, and one or more other. electrodes represented schematicallyas 9. While a particular form of oscilloscope tube has been shownschematically in Fig. 1, any of the conventional forms may be employedin the prac-. tice of this invention. For example, instead of theelectrostatic deflector plates 6, electromagnectic deflector coils maybe used or the beam may be deflected along one axis by electrostaticmeans and along the other axis at right angles thereto byelectromagnetic means.

Connected to the cathode l and intensity control electrode 8 is avariable low frequency cali- 'brated oscillator ill- The voltage outputof this oscillator is so adjusted as to periodicall blank or release thebeam generated by the electron gun of oscilloscope tube 3.

As is customary in oscilloscope practice, amplifiers may be employed inany of the deflector or control electrode channels. For example, thestandard frequency source may be amplified by an amplifier l, the outputof which is coupled to the corresponding deflector plates through acoupling condenser and resistor as indicated in Fig. 1. Similarconnections through an amplifier 5 are shown for the unknown frequencysource 2.

The production of Lissajous figures by the means of a cathode rayoscilloscope is so well known that it need not be described in detail.It need only be remembered that regardless of the two frequenciescombined on the deflector plates of the oscilloscope, the pattern willalways be reentrant and where the frequencies approach equality thepattern will repeat itself at a rate equal to the difference between thetwo frequencies. In so far as the effect on the eye is concerned, thispattern becomes entirely unintelligible when the frequency difference isgreater than 10 to 15 cycles per second and the frequency ratio must berepresented by large numbers. The

Lissajous figures ll represented in Fig. 2 are for I the special easeswhere the two frequencies are equal but differing in phase as indicatedby the -It is a well-known fact that where the frequency difierence isin the order of 2 or 3 cycles per second, or less, and the frequencyratio contains large integers, the discrete Lissajous figures as shownin Fig. 2 appear to successively form'one after another as ifa hoop werebeing rotated voltages, the standard voltage being used as a referencevector and designated with a reference character S, while the unknownvoltage is rep-- resented by the vector 'U. The curved arrow adjacentthe unknown voltage vector U represents counterclockwise rotation withrespect to the standard frequency reference vector S.

The resulting image'on the oscilloscope screen will appear similar tothat shown in Fig, 3 whenever the frequency ratio .contains largenumbers the eye.

and the frequency difference exceeds about 10 cycles per second. In theprevious simplified description of the production of these Lissajousfigures, it must be remembered that figures ll shown in Fig. 2 are thefigures that would be produced should the phase angle remainv fixed. attheflangles indicated. The actual path of the beam is a little morecomplicated by reason of the fact that the phase angle is uniformlyshifting. If the frequencies arelarge enough, the pattern may becomereentrant at a, rate more rapid than the persistence of vision andconsequently the reentrant Lissajous figure'appears stationary to Whenthe two frequencies are nearly equal, the pattern becomes reentrant oncefor each complete cycle of frequency difference between the standard andunknown frequency sources. Consequently, if the trace canbe blanked outonce for each cycle of frequency difference,

and, for the major portion of' that cycle, substantially only one epochof the complicated Lissajous figure l3 -of Fig. 3 willappear on the.screen-as shown at M- in Fig. 4. Furthermore,

if this blanking frequency differs slightly from the differencefrequency between, the standard and unknown sources, this figure I4 ofFig. '4

will not remain stationary but will'seem to go of standard frequency toone of the beam defleeting means, means connecting the source of unknownfrequency to the other beam ,d efl ecting means, means connecting thecalibrated variable frequency source to the beam intensity controlelectrode, and a bias source for said beam in-v Y tensity controlelectrode, said source being adthrough the various phases indicated at lI in Fig.

2. actly equal to the difference frequency, this stroboscopic image [4shown in Fig. 4 will be stationary.

The action just described is preferably produced by the apparatus ofthisinvention by em-- ployin a variable low frequency calibrated oscillatorID as shown in Fig. 1 which produces a peaked output as indicated atIll. The circuits When the blanking frequency is made ex-' arepreferably so arranged that the beam is blanked out except during thepeaked periods of 1 the peaked voltages Ill from the variable lowfrequency calibrated oscillator l0. By reason of the stroboscopic actionproduced by this low fre' quency oscillator, it is evident that thefrequency thereof will be equal to the difference frequency betweenthestandard source I and the unknown frequency source 2 when thestroboscopic figure is made stationary; Consequently the unknown andstandard frequency sources may be applied to the beam deflectorelectrodes of the oscilloscope 3 and the variable low. frequencycalibrated oscillator adjusted until the stroboscopic image appearsstationary'on the screen as shown at 84 in Fig. 4. The frequency as thenindicated by the calibrated low frequency oscillator will be equaljusted to normally suppress the beam until the positive peak voltagefrom said variable frequency source exceeds a predetermined limitwhereupon said beam may be interrupted at a frequency.

equal to the difference between the standard and unknown frequencies.

3. An apparatus for ,comparing the" frequency of a source ofunknownfrequency with a source of standard frequency comprising incombination a cathode ray oscilloscopeincluding horizontal and verticalbeam deflectin means and a-beam intensity contro1 electrode, acalibratedvariable frequency source of the type 1.-which produces peaked'voltagepulses of short'duration'relative v t0 the period of the. cycle, meansconnecting said source of standard frequency to one of the beamdeflecting means, means connecting the source of j unknown frequency tothe other beam deflecting-3 means and means connectin thecalibratedvariable frequency source to the beam intensity controlelectrode, whereby said beam is caused to form a Lissajous figure onlyduring the short duration of said peak voltage from the variablefrequency source.

4. An apparatus for comparin t frequency of a Source of unknownfrequency with a source of standard frequency, comprising in combinationa cathode ray oscilloscope including hori- Zontal and-vertical beamdeflecting means and a beam intensity contro1 electrode,'a calibratedvariable frequency source of the type whichproduces peakedvoltage pulsesof short duration relative to the period of the cycle, means connectingsaid source of standard frequency to one of the beam deflecting means;meansconnecting the source of unknown frequency to the other to thediiference frequency between the standard and the unknown sources.

What is claimed is:

1. The method of comparing an unknown frequency with a standardfrequency comprising causing said frequencies to cooperate to produce aLissajous figure, stroboscopically interrupting the production of saidfigure by 'employinga variable frequency of known magnitudaand,

adjusting the magnitude of said variable frequency until saidinterrupted figure appears sta- I tionary, whereupon the magnitude ofsaid variable frequency.is a measure of the difference between thestandard and unknown frequencies. 2. An apparatus for comparing thefrequency of a source of unknown frequency with'a source f standardfrequency comprising in combination a cathode ray oscilloscope includinghorizontal and vertical beam deflecting means and a beam intensity,control electrode, a calibrated variable frequency source, meansconnecting said sou'rce beam deflecting means, means" connecting thecalibrated variable frequency' 'source to the beam intensity controlelectrode and a bias source for said beam intensity control electrode,saidsource being adjusted to normally suppress the beam until thepositive peak voltage from said variable frequency source exceeds apredetermined limit, whereby said beam may be caused to produce theLissajous figure onl during said peakvoltage periods of short duration.

ROBERT T. ADz s.

REFERENCES CITED The following references are of record nfile of thispatent:

. UNITED STATES- PATENT Von Duhn May 27, 1941

